360 research outputs found
Chern-Simons S-Brane Solutions in M-theory and Accelerating Cosmologies
We construct an intersecting S-brane solution of 11-dimensional supergravity
for which the contribution of the Chern-Simons term to the field equations is
non-zero. After studying some of its properties, we consider three different
compactifications (each with 3 separate subcases) of this system to
4-dimensions. Two of these give accelerating cosmologies, however their
expansion factors are of order unity. We also find two static versions of this
configuration and its dimensional reduction to type IIA theory.Comment: 18 pages, v3: a new static solution and references added, v4:
references added, to appear in JHE
Non-equilibrium dynamics of stochastic point processes with refractoriness
Stochastic point processes with refractoriness appear frequently in the
quantitative analysis of physical and biological systems, such as the
generation of action potentials by nerve cells, the release and reuptake of
vesicles at a synapse, and the counting of particles by detector devices. Here
we present an extension of renewal theory to describe ensembles of point
processes with time varying input. This is made possible by a representation in
terms of occupation numbers of two states: Active and refractory. The dynamics
of these occupation numbers follows a distributed delay differential equation.
In particular, our theory enables us to uncover the effect of refractoriness on
the time-dependent rate of an ensemble of encoding point processes in response
to modulation of the input. We present exact solutions that demonstrate generic
features, such as stochastic transients and oscillations in the step response
as well as resonances, phase jumps and frequency doubling in the transfer of
periodic signals. We show that a large class of renewal processes can indeed be
regarded as special cases of the model we analyze. Hence our approach
represents a widely applicable framework to define and analyze non-stationary
renewal processes.Comment: 8 pages, 4 figure
Regular S-Brane Backgrounds
We construct time-dependent S-brane solutions to the supergravity field
equations in various dimensions which (unlike most such geometries) do not
contain curvature singularities. The configurations we consider are less
symmetric than are earlier solutions, with our simplest solution being obtained
by a simple analytical continuation of the Kerr geometry. We discuss in detail
the global structure and properties of this background. We then generalize it
to higher dimensions and to include more complicated field configurations -
like non vanishing scalars and antisymmetric tensor gauge potentials - by the
usual artifice of applying duality symmetries.Comment: 22 pages, 3 figures. Typos in eq.(2.6) correcte
Particle Production on Half S-brane
In this paper we will study quantum field theory of fluctuation modes around
the rolling tachyon solution on non-BPS D-brane effective action. The goal of
this paper is to study particle production during the decay of non-BPS D-brane
and explore possible relation with minisuperspace calculation. We find that the
number of particles produced on half S-brane exponentially grows for large time
which suggests that linearised approximation breaks down and also that
backreaction of fluctuation field on classical solution should be taken into
account.Comment: 24 pages,v1.references adde
Intersecting S-Brane Solutions of D=11 Supergravity
We construct all possible orthogonally intersecting S-brane solutions in
11-dimensions corresponding to standard supersymmetric M-brane intersections.
It is found that the solutions can be obtained by multiplying the brane and the
transverse directions with appropriate powers of two hyperbolic functions of
time. This is the S-brane analog of the ``harmonic function rule''. The
transverse directions can be hyperbolic, flat or spherical. We also discuss
some properties of these solutions.Comment: 12 pages, Latex, a reference adde
De novo Renal Transplantation after Kaposi Sarcoma: Favorable Outcome in a Patient Receiving Sirolimus and Mycophenolate-Based Immunosuppression
Immunosuppressive treatment increases the risk of infection and malignancy in organ transplant recipients. We report on a 42-year-old male renal transplant recipient who lost his first graft after reduction of immunosuppressive treatment due to Kaposi sarcoma and who successfully underwent a second renal transplant 10 years later. The patient's current treatment consists of low-dose prednisone, and the two antiproliferative immunosuppressants mycophenolate mofetil and rapamycin. 4.5 years after his second transplant, the serum creatinine is 1 mg/dl and the patient has no signs of recurrent disease
Localized Intersections of Non-Extremal p-branes and S-branes
A class of solutions to Supergravity in 10 or 11 dimensions is presented
which extends the non-standard or semi-local intersections of Dp-branes to the
case of non-extremal p-branes. The type of non-extremal solutions involved in
the intersection is free and we provide two examples involving black-branes
and/or D-\bar{D} systems. After a rotation among the time coordinate and a
relatively transverse radial direction the solutions admit the interpretation
of an intersection among D-branes and S-branes. We speculate on the relevance
of these configurations both to study time dependent phenomena in the AdS/CFT
correspondence as well as to construct cosmological brane-world scenarios
within String Theory admitting accelerating expansion of the Universe.Comment: 31 pages, latex file; v2: typos corrected and references adde
On Winding Branes and Cosmological Evolution of Extra Dimensions in String Theory
We consider evolution of compact extra dimensions in cosmology and try to see
whether wrapped branes can prevent the expansion of the internal space. Some
difficulties of Brandenberger and Vafa mechanism for decompactification are
pointed out. In both pure Einstein and dilaton gravities, we study cosmology of
winding brane gases in a continuum approximation. The energy momentum tensor is
obtained by coupling the brane action to the gravity action and we present
several exact solutions for various brane configurations. T-duality invariance
of the solutions are established in dilaton gravity. Our results indicate that
phenomenologically the most viable scenario can be realized when there is only
one brane wrapping over all extra dimensions.Comment: 17 pages, RevTex4, 2 figures, a paragraph added, to appear in
Classical and Quantum Gravit
BPS pp-wave brane cosmological solutions in string theory
We construct time dependent BPS pp-wave brane solutions in the context of
M-theory and type II supergravity. It is found that N-brane solutions we
considered satisfy the crossing rule as S-brane solutions but 1/8 supersymmetry
remains. By applying them to the cosmological setting, inflationary solutions
are obtained. During this inflation, the size of the extradimensions becomes
smaller than our four-dimensional spacetime dynamically. We also discuss the
mechanism for terminating this inflation and recovering the hot big-bang
universe.Comment: 10 page
Hyperbolic Space Cosmologies
We present a systematic study of accelerating cosmologies obtained from
M/string theory compactifications of hyperbolic spaces with time-varying
volume. A set of vacuum solutions where the internal space is a product of
hyperbolic manifolds is found to give qualitatively the same accelerating
four-dimensional FLRW universe behavior as a single hyperbolic space. We also
examine the possibility that our universe is a hyperbolic space and provide
exact Milne type solutions, as well as intersecting S-brane solutions. When
both the usual 4D spacetime and the m-dimensional internal space are
hyperbolic, we find eternally accelerating cosmologies for , with and
without form field backgrounds. In particular, the effective potential for a
magnetic field background in the large 3 dimensions is positive definite with a
local minimum and thus enhances the eternally accelerating expansion.Comment: 33 pages, 2 figures; v2 refs added; v3 minor change in text, JHEP
versio
- …